1. Field of the Invention
The present invention relates to an optical pickup for reading recorded information from an optical disk, and more particularly to an optical pickup of a compatible type which can be used for two types of optical disks having different thicknesses up to their recording surfaces, such as a digital video disk (DVD) and a compact disk (CD).
2. Detailed Description of the Related Art
The DVD is an optical disk capable of large volumes of digital information, and allows digital information, such as moving pictures, computer information and the like, to be recorded on a disk having the same 12 cm diameter as the CD with a recording density approximately eight times or more as large as that of the CD. In the DVD, various devices have been made to attain such a high-density recording.
Namely, to increase the storage capacity, the wavelength of a laser light source used is set at 650 nm which is shorter than 780 nm for the CD, and the numerical aperture NA of an objective lens is set at 0.6 which is larger than 0.45 for the CD, thereby attaining high-density recording of about 5 G bytes on one surface of the disk.
As described above, since the DVD allows higher-density recording than the CD, it is necessary to make the spot diameter of a laser beam for reading pit information much smaller than that in the case of the CD. The spot diameter of the laser beam is proportional to the wavelength .lambda. of the laser used, and is inversely proportional to the numerical aperture NA of the objective lens. In the DVD, the spot diameter is made small by using a laser light source having a short wavelength and an objective lens having a large numerical aperture NA.
However, in case that the wavelength .lambda. of the laser is made short for the DVD and the numerical aperture NA of the objective lens is made large, wavefront aberration (mainly coma-aberration) occurs even if the disk is tilted even slightly, and the margin for a so-called tilt angle, i.e., the angle at which the disk surface is tilted from a perpendicular direction to the optical axis of the objective lens of the optical pickup, becomes small.
The DVD and the CD are optical disks of the same form, and it is desirable to allow the CD to be reproduced by a DVD player. In addition, since the substrate of the disk of the CD is thicker than that of the DVD (e.g., 1.2 mm for one side of the CD as compared to 0.6 mm for one side of the DVD), if the CD is reproduced by using a pickup optimally designed for the DVD, wavefront aberration (mainly spherical aberration) occurs due to the difference in the thickness of the substrate, enlarging the spot diameter of the laser beam. Consequently, there arises the problem that if the optical pickup which is optimally designed for the DVD is used as it is, the information of the CD cannot be read.
For this reason, various methods have been conventionally proposed, including a method in which two exclusive-use objective lenses are prepared for respectively reproducing the DVD and the CD, and the lens is changed over depending on the disk, a method in which a correcting lens is inserted in a collimator portion, and in which an aberration due to the disk is corrected, and a method in which a bifocal lens using a hologram is used as the objective lens.
However, with the method in which two objective lenses are used and the method in which the correcting lens is used, there are drawbacks in that the mechanism is complicated, and that it occupies a large space and is therefore unsuitable for making the apparatus compact in size.
In addition, with the method in which the bifocal lens using a hologram is used, there are drawbacks in that the light utilizing ratio is low due to the effect of diffraction and the like, and that since a multi-beam is used, the effect of interference is liable to occur.
Further, if an arrangement is provided so as to allow both the DVD and the CD to be used, it is difficult to simultaneously effect correction with respect to the tilting of the disk (hereafter referred to as the tilt correction), so that it is necessary to provide a tilt correcting means separately.
If the tilt correcting means is provided separately, the optical pickup apparatus becomes large in size, and the cost becomes high.
For this reason, to overcome these drawbacks, an optical pickup of a compatible type for effecting the changeover between the DVD and the CD, and the tilt correction by using a liquid crystal device has been proposed by the present applicant in Japanese Patent Application No. 109795/1996.
This apparatus is arranged such that a liquid crystal device is disposed in the optical path of the optical pickup, voltage is applied to the liquid crystal device in correspondence with the detected tilt angle to impart a phase difference to the passing bundle of rays, thereby correcting the effect of wavefront aberration ascribable to the tilt angle.
However, the conventional liquid crystal device has not been optimized with respect to the wavefront aberration ascribable to the tilt angle, and has been impossible to completely eliminate the wavefront aberration. Hence, the effect of the wavefront aberration has remained, and it has been impossible to obtain favorable reproducing characteristics.